1. Field of the Invention
The present invention relates to a composite film comprising a para-oriented aromatic polyamide and a low dielectric resin.
2. Description of the Related Art
The high-speed signal processing and digitizing techniques have been highly demanded for high-performance electronic instruments. A laminated board including a non-woven fabric of aromatic polyamide (hereinafter may be referred to as aramid) as a base substrate has characteristics such as low dielectric constant, light weight, and low linear thermal expansion coefficient. Various applications of the laminated board have been developed especially in the field of multi-layer printed circuit boards. Although a polyimide film has been used for the flexible boards, further reduction in dielectric constant is required for high-speed signal processing.
Aromatic aramids are mainly divided into two groups; that is, meta-oriented aromatic polyamides (hereinafter may be referred to as meta-aramid) and para-oriented aromatic polyamides (hereinafter may be referred to as para-aramid). Paper made of meta-aramid has practically sufficient strength and tear strength, but the large linear thermal expansion coefficient makes the paper unsuitable as a printed circuit board. The para-amide fibers have excellent properties, such as high strength, high stiffness, high heat resistance, and low linear thermal expansion coefficient. On the other hand, since para-aramid does not melt, para-aramid paper manufactured from para-aramid pulp does not have agglomerated portions. The para-aramid paper accordingly has low strength and difficulties in handling for the printed circuit board. Thus, heat-resistance resins are widely used as a binder of para-aramid paper or non-woven fabric.
For example, U.S. Pat. No. 5,314,742 describes that a non-woven fabric composed of meta-aramid fibrils and para-aramid flocks has a low linear thermal expansion coefficient and is effectively used as the base substrate of a laminated board. JP-A-5-327148 describes that cloth, paper, and non-woven fabric containing para-aramid fibers at least 50% as a base substrate decreases the linear thermal expansion coefficient in the planar direction. However, it is difficult to obtain a non-woven fabric of uniform formation, and further improvement has been demanded. The dielectric constant at 1 MHz of para-aramid is 3.5, and the conventional technique of simply impregnating para-aramid with epoxy resin to produce a prepreg for a printed circuit board can not sufficiently reduce the dielectric constant.
Several methods of adding a low dielectric constant fluororesin to a base substrate for the purpose of lowering the dielectric constant are disclosed. EP-A-320901 discloses a method for producing a laminated board comprising a fluororesin reinforced with a woven fabric of a nonwoven fabric of aramid. However, the diameter of the fiber composing the above woven and nonwoven fabric is about 15-20 .mu.m, and recently, a base substrate having more uniform formation and textile is desired in view of the necessity of fine patterning during the production of printed circuit board.
Similarly, for lowering the dielectric constant, JP-A-4-55437 discloses a woven or nonwoven fabric comprising aramid fiber impregnated with a dispersed solution of fluororesin and glass beads, and JP-A-2-268486 discloses a board material using a base substrate prepared by paper-making a mixture of fluororesin fiber and organic fiber. However, either method loses a required high uniformity of the base substance for fine patterning.
HP-B-1-33493 discloses a method that an aqueous dispersion of fluororesin applied on the surface of an aromatic polyamide film is dried and heat-treated to produce a fluororesin layer on a single face or both faces of the aromatic polyamide film.
Methods of producing a laminated film having low dielectric constant as a whole by laminating a fluororesin-containing layer on a layer comprising a thermosetting resin and a base substrate (JP-B-6-94206), or on a layer comprising a combination of a fluororesin and fiber-reinforcing material (JP-A-3-273695).
These laminated films, being made of different materials and layers, are not uniform as a whole film.
Moreover, a method of further laminating a low conductive layer cannot make a printed circuit board thin.
As a low dielectric constant film, Polymer Preprints (Vol. 37, No. 1, p160, 1996, Division of Polymer Chemistry, Inc. American Chemical Society) describes an aromatic polyester film containing pores. It is proposed that pores composed of air phase of low dielectric constant are formed in the aromatic polyester having a low linear thermal expansion coefficient in order to lower the dielectric constant of the polyester. However, the pores formed in the film may cause a problem such as contamination of an etching solution or flow-in of the water in the processing procedure of the printed circuit board.
An object of the present invention is to provide a composite film that is light in weight, has a low dielectric constant, a low linear thermal expansion coefficient, homogeneous structure, and favorable mechanical strength for a printed circuit board, as well as to provide a base substrate for a flexible printed circuit board using such a film.
Another object of the present invention is to provide especially a porous composite film and a prepreg prepared by impregnating the porous composite film with a thermoplastic resin and/or a thermosetting resin.
Still another object of the present invention is to provide a printed circuit board having a laminated board using such a prepreg.